PP2000 does 40 damage, dropping to 25 at 10 meters, and dropping to 20 at 20 meters, and holds 20 damage.
UMP does 40 damage, dropping to 35 at 15 meters, and holds 35 damage.
Silencer SIGNIFICANTLY reduces PP2000 damage beyond 5 meters.
Silencer ONLY SLIGHTLY reduces UMP damage beyond 10 meters. The UMP is actually the only weapon in the game that the silencer has almost no impact on.
Conclusion: With Stopping Power, either gun will kill in 2 hits at close range. At mid-long range, UMP has the advantage. With a silencer, UMP also has the advantage.
For an all-purpose, silenced weapon, I would use the UMP.